The present invention relates to an improvement in an amplifier circuit, and more particularly, to an improvement in a high frequency characteristic.
FIG. 24 shows a conventional amplifier circuit. In FIG. 24, denoted at 70 is a current supply part, denoted at SW1, SW2 . . . SWn are switch circuits, and denoted at R1, R2 . . . Rn are magnetic resistance elements whose resistance values change due to an external factor. The magnetic resistance elements R1 to Rn correspond to said switch circuits SW1 to SWn, respectively. Denoted at 71 is a capacitance, denoted at 72 is a feedback circuit, denoted at 73 is an amplifier, and denoted at 74 is an output terminal of the amplifier circuit. By shorting one of the plurality of the switch circuits SW1, SW2 . . . SWn (e.g., SW1), the magnetic resistance element R1 which corresponds to the shorted switch circuit is selected. At this stage, a current is supplied to said selected magnetic resistance element R1 from the current supply part 70 through said selected switch circuit SW1. If the resistance value of the selected magnetic resistance element R1 changes due to an external factor, a voltage which is developed across the magnetic resistance element R1 changes, and this voltage change is supplied to the amplifier 73 through the capacitance 71. The voltage change thus supplied is amplified by the amplifier 73, and an amplified voltage is outputted at the output terminal 74.
However, said conventional amplifier circuit has the following disadvantages.
That is, since a signal which expresses the voltage change at the selected magnetic resistance element is coupled with the amplifier 73 by the capacitance 71, the capacitance 71 and an input impedance of the amplifier 73 create a high-pass filter, and this filter reduces a gain in a low region and hence a low region frequency characteristic.
Further, while the feedback circuit 72 reduces a noise which is included in an output from the amplifier 73, because of a delay created by the feedback circuit 72, the output oscillates easily in a high frequency region, and thus, the output is not enough large to deal with oscillation in the high frequency region. Hence, it is impossible to maintain a high gain in a higher frequency region.
Still further, since the switch circuits SW1 to SWn are connected to the magnetic resistance elements R1 to Rn in series, when the resistance value of a selected magnetic resistance element changes, a resistance component and an inductance component of the shorted switch circuit causes a voltage across the shorted switch circuit to change. This voltage change is combined with the change in the voltage across the magnetic resistance element and supplied to the amplifier 73. As a result, an output from the amplifier 73 includes an error. Thus, amplification performed by the amplifier 73 is influenced by the switch circuit.